The present invention relates to an implant for insertion into and permanent anchorage in human skeletal or bone tissue. More specifically, the invention concerns an implant having a screw-threaded anchoring structure comprising a first cylindrical section of a first diameter and a second cylindrical section of a second diameter, said second diameter being less than said first diameter.
The invention is particularly, although not exclusively, concerned with an implant of this type which takes the form of a femur fixture for a hip-joint pros* thesis.
Implants of this type in the form of femur fixtures for a hip-joint prosthesis are known from Applicant""s prior International patent application publications WO93/01769, WO93/16663 and WO97/25939 with the first and second cylindrical sections being provided with external screw threads for engaging the bone tissue of the femur. The screw threaded first and second cylindrical sections of the femur fixture-disclosed in WO93/01769 are assembled together in the femur by firstly inserting the second cylindrical section medially into the neck of the femur from beneath the greater trochanter and then inserting the first cylindrical section into the neck laterally through the resected section left after resection of the head of the femur. The screw threaded first and second cylindrical sections of the femur fixture disclosed in WO93/16663 and WO97/25939, on the other hand, are integrally formed or pre-assembled prior to anchorage of the fixture in the femur neck by screwing the fixture into the femur neck laterally through the resected section left after resection of the femur head.
In the femur fixtures disclosed in WO93/01769, WO93/16663 and WO97/25939, the first cylindrical section steps into the second cylindrical section. A drawback of this type of implant is the amount of stress present between the bone and the implant following implantation.
Thus, the object of the present invention is to provide an implant where the above mentioned drawback is reduced.
This and other objects are achieved according to the present invention by providing an implant having the features defined in the independent claim. Preferred embodiments are defined in the dependent claims.
According to the present invention there is provided an implant for insertion into and permanent anchorage in bone tissue, comprising an intraosseous anchoring structure of a generally circular cross-section, said anchoring structure comprising a first cylindrical section of a first diameter and a second cylindrical section of a second diameter, said second diameter being less than said first diameter, said first and second cylindrical sections each being provided with a screw thread profile. The implant is characterised in that said anchoring structure comprises a tapered connecting section provided between and interconnecting said first and second cylindrical sections.
The implant of the invention may be an orthopaedic implant, for example a femur fixture for a hip-joint prosthesis as in the embodiment hereinafter to be described.
Thus, the present invention is based on the advantageous idea of providing an implant of the above-mentioned type with a tapered connecting section between the first and second screw-threaded, cylindrical sections.
The provision of a tapered connecting section would overcome the drawback mentioned above and provide a number of additional advantages. First, the stresses induced by the sharp, step-wise transition present in the prior art implants between the cylindrical sections of differing diameters is radically reduced using the tapered connecting section of the present invention.
A further advantage resulting from the provision of a tapered connecting section when the implant is used for implantation in a cavity of corresponding shape formed in bone tissue, is that the insertion of the implant is facilitated. This is because the distal end of the implant can be guided by the tapered section of the cavity which is arranged for interacting with the tapered connecting section. Thus, the cavity will have no surface facing directly opposite the insertion direction of the implant, as is the case with the prior art implants (see item 50 of FIG. 5 in WO 97/25939).
A still further advantage is that the provision of a tapered connecting section provides a wedging effect during implantation of the implant into bone tissue. This wedging effect improves the short and long term stability of the implant following said implantation. This is mainly due to the radial force component of the normal contact force between the implant and the cancellous bone tissue that surrounds the connecting section upon implantation.
If the longitudinal force exerted by the surrounding tissue on the implant of the invention and on the above-mentioned prior art implants, following implantation of the implant, is essentially the same, then the transversal force on the implant of the invention will be greater compared to the prior art stepped implants. This is because the longitudinal force is carried by the inclined surface of the tapered connecting section, whereby the resulting transversal force will have a radial force component. This radial force component is not present in the prior art implants since only the radial end portion (e.g. item 10 of FIG. 1 in WO 97/25939) of the proximal cylindrical section (e.g. item 2 of FIG. 1 in WO 97/25939) carries the corresponding longitudinal force.
While the overall transversal force is increased, the actual force per surface unit is not necessarily increased. This is due to the fact that the contact surface carrying the longitudinal contact forces will be considerably greater with the tapered connecting section of the present invention as is the case with said radial end portion of the prior art implants.
Also, the provision of the inventive tapered connecting section improves the integration between the implant and the cancellous bone tissue (termed xe2x80x9cosseointegrationxe2x80x9d in the art). This is mainly due to the shape of the contact surface of the connecting section, but also to the increased overall contact force exerted on the implant by the cancellous bone tissue.
Ordinarily, the first cylindrical section is disposed proximally of the second cylindrical section with the taper of the connecting section inclining inwardly in the distal direction, as in the embodiment hereinafter to be described.
Preferably, the tapered connecting section has a frusto-conical profile, even though other tapering shapes are conceivable without departing from the scope of the present invention.
According to preferred embodiments of the invention, the diameter of the first cylindrical section is adapted to the actual size and shape of the femur of the particular patient for whom the implant is intended. Thus, the diameter of the first cylindrical section can vary considerably. However, the diameter of the second cylindrical portion is preferably dimensioned to be within a short, limited range. Thus, the flank angle of the connecting section may vary in dependence of the actual dimensions of the first and second cylindrical sections. Preferably, the flank angle can be varied in the range of 10xc2x0-50xc2x0, and more preferably in the range of 200-400.
Furthermore, the longitudinal extension of the connecting section is preferably in the range of 5-15 mm, preferably in the range of 7-11 mm.
Advantageously, the end of the tapered connecting section interfacing the first cylindrical section has essentially the same diameter as the first cylindrical section. Likewise, the end of the tapered connecting section interfacing the second cylindrical section advantageously has essentially the same diameter as the second cylindrical section.
According to preferred embodiments of the invention, the connecting section is at least partly provided with a roughened surface. This would even further promote the osseointegration process at the transition area between the cylindrical sections. The roughened surface could be achieved through blasting, preferably grit-blasting, etching, or the like. Alternatively or additionally, a circumferentially oriented roughness, preferably machined, could be provided on the connecting section. Such circumferentially oriented roughness could for instance be provided in the form of grooves, beads, tracks, or screw threads.
According to preferred embodiments of the invention the tapered connecting section is at least partly provided with a screw thread profile. The screw thread profile of the tapered connecting section is preferably different from the screw thread profiles on the first and second cylindrical sections. Preferably, the height of the screw thread profile of the connecting section is less than the height of the screw thread profiles of the cylindrical sections. Advantageously, the screw thread profile of the connecting section are in the form of microthreads having a height of less than 0.3 mm, preferably in the range of 0.1-0.25 mm, even more preferably about 0.2 mm.
Other differences in screw thread profiles are also conceivable, such as the screw thread profile of the connecting section having a pitch less than the pitch of the screw thread profiles of the cylindrical sections.
Alternatively, the screw thread profile of the connecting section is essentially the same as that of the cylindrical sections.
Where the circumferential roughness is provided in the form of beads or tracks, the height of said circumferential roughness is preferably less than 0.3 mm, more preferably in the range of 0.1-0.25 mm, and even more preferably approximately 0.2 mm.
Alternatively, at least part of the surface or the entire surface of the connecting section may be left smooth, or even polished.
In an embodiment of the invention, such as the one hereinafter to be described, one or more self-tapping cutting recesses are provided at least in part on the tapered connecting section. Said cutting recesses preferably being equi-spaced and circumferentially arranged.
According to preferred embodiments of the invention, the implant also comprises a tapered proximal section interconnecting the first cylindrical portion with a head section of the implant. The tapered proximal section provides increased contact between the implant and surrounding cortical bone tissue and improves the stability of the implant when anchored in bone tissue.
The tapered proximal section advantageously has a frusto-conical shape with a flank angle that is preferably in the range of 8xc2x0-150, and even more preferably in the range of 100-130, and even more preferably approximately 12xc2x0. Preferably, the end of the tapered proximal section abutting the first cylindrical section has a diameter essentially equal to the diameter of the first cylindrical section.
Preferably, the surface of the tapered proximal section is provided with a circumferentially oriented roughness, for instance in the form of circumferential beads or screw threads. The height of the beads or screw threads is preferably no greater than 0.3 mm, more preferably in the range of 0.1-0.25 mm, and even more preferably approximately 0.2 mm.
Preferably, said beads or screw threads engage with the cortex 34 of the femur neck at the resected surface. As a result, a stronger short term anchorage of the implant is provided. Also, a stronger long term anchorage is provided due to the improved osseointegration between the tapered proximal section and the surrounding bone tissue.
According to embodiments of the present invention, the above-mentioned head section is provided with a collar abutting the tapered proximal section, which collar delimits the insertion of the implant into bone tissue. Preferably, the surface of the collar facing the proximal section is inclined inwardly so as to mate with a resected bone tissue surface that has been given a correspondingly inclined shape. Preferably, the angle of inclination is within the range of 100-200, preferably approximately 15xc2x0. Alternatively, the surface of the collar facing the proximal section is given a concave shape, so as to mate with a convex bone tissue surface. Thereby, an improved contact between the implant and the bone surface can be obtained.
Preferably, said collar surface is provided with radially spaced circular beads or grooves for increasing the stability of the inserted implant and promote the osseointegration between the implant and the bone tissue. Preferably, these beads have a height in the range of 0.1-0.5 mm, preferably in the range of 0.2-0.4 mm, and even more preferably approximately 0.3 mm.
By way of example, an embodiment of the invention will now be described with reference to the accompanying Figures of drawings.